1. Field of the Invention
This invention relates to apparatus and method for surveying and logging deviated boreholes and, more particularly, but not by way of limitation, to apparatus and method used for positioning surveying, logging, and other tools in highly deviated boreholes and boreholes with a short radius curve.
2. Setting of the Invention
In order to enhance the recovery of subterranean fluids, such as oil and gas, it is sometimes desirable to steer the direction of the well bore or borehole. In an oil producing formation or strata which has limited vertical depth and relatively greater horizontal extent, a well which extends laterally or horizontally through the oil producing formation may be more productive than one extending vertically. In order to create a deviated, e.g., skewed or horizontal, borehole it is necessary to steer the drilling bit at the end of the drill string from the general vertical orientation in which such wells are normally drilled to a horizontal or lateral orientation.
In steering a drill bit from a vertical orientation to a deviated orientation, it is necessary to deflect the drill bit from the generally vertical borehole to the deviated direction. The initial deflecting step is also known as "kicking off" the well or borehole. It is typical to first drill a vertical borehole and then attempt to deflect the drill bit and drill string through the wall of the vertical borehole. Normally the deviated section of the borehole will "kick off" from the vertical section of the borehole with a curved section of borehole. The borehole will be "curved" until the desired inclination is obtained and then the borehole will be extended as desired in the inclined (or "deviated"), lateral direction. A borehole or a section of a borehole is considered "highly" deviated when an instrument or tool will not travel into the deviated section under the instrument's own weight, otherwise the borehole is considered to be about vertical. This usually occurs when the deviated section has an inclination of approximately 55 degrees or more, measured from a vertical axis. Highly deviated boreholes often are initiated by a curved section of borehole having a short radius ("short radius curve"), i.e., having a radius of curvature between ten and 100 feet.
Several types of tools have been developed to kick off the drill bit and drill string from the vertical section of the borehole and create the deviated section of the borehole. The whipstock, razorback, eccentric collar, and similar tools are used to guide the direction of the drill bit as the drilling progresses in the deviated section of the borehole. As drilling progresses in the deviated section, it is essential to monitor the direction, i. e., north, south, east, west, as well as the inclination of the deviated section in order to determine when directional changes need to be made. Various surveying instruments have been developed which may be placed in the deviated section of the borehole in order to monitor or "survey" the direction and inclination of the borehole. One tool is disclosed in U.S. Pat. No. 4,807,717 which utilizes a collar on the logging tool to be caught by a special collar catcher on the end of the drill string. The borehole is then logged while the drill string is withdrawn from the borehole. Other systems that are commercially available include Institut Francais Du Petrole's "SIMPHOR", Halliburton (Gearhart) Logging Service' s "TOOL PUSHER", Atlas' "SLANT-HOLE EXPRESS" and Welex's "PLS".
However, problems are encountered when attempting to survey, log, or otherwise insert tools into boreholes, particularly highly deviated boreholes and short radius curves. Highly deviated boreholes present a surveying problem because of their high angle of deviation and the high curvature of the curved section. Vertical boreholes, low deviation boreholes, and any initial, low deviation section of the curved section of a highly deviated borehole or short radius curve can be surveyed by lowering the surveying instrument via wireline into the borehole; but the instrument will not slide into the deviated section under its own weight beyond inclinations of about 55 degrees from vertical. At inclinations greater than 55 degrees, the tool or instrument must be pushed or somehow forced into the deviated section or lateral extension of the borehole.
A common prior method for surveying short radius curves or highly deviated boreholes conveys the surveying instrument at the end of a non-magnetic (such as aluminum or stainless stell) spacer attached to a string of flexible steel drill pipe (or collars). The method entails removing the drill string from the borehole, attaching the survey assembly to the drill string, tripping (moving) the drill string and survey assembly into and out of the borehole, removing the survey assembly, and tripping the drill string back into the borehole. This makes surveying, logging, or otherwise placing tools in a highly deviated or short radius curve borehole a time consuming operation which detracts from the efficiency of the drilling of highly deviated boreholes. The most common complaint about short radius or highly deviated drilling is the inordinate time devoted to surveying the borehole.
Flexible or wiggly pipe is made specifically for drilling and is not particularly appropriate or ideal for moving instruments or tools into and out of boreholes. In surveying applications, wiggly pipe is unnecessarily heavy, is overdesigned strength-wise, and its ability to conduct fluid is not needed. Also, wiggly pipe is a relatively expensive specialty drilling tool which undergoes needless wear and tear and is exposed to unnecessary risk of loss downhole whenever it is used for surveying highly deviated boreholes.
Another problem encountered with highly deviated boreholes or boreholes having short radius curves is that there are no survey instruments sufficiently small to be run through the bore of the flexible pipe or wiggly collars used for drilling the short radius curve. When a deviated borehole or short radius curve is being drilled, it is desirable to take surveys immediately above the drill bit as the curve drilling progresses in order to: (1) determine the best orientation of the kick off tool for directional control, (2) to tell if the curve drilling is progressing normally, and (3) to determine when to pull the curve drilling assembly and replace it with a lateral drilling assembly. In practice, the curve drilling assembly is pulled from the borehole one or more times during drilling of the deviated section in order to perform these functions. Each time this is done, valuable rig time is wasted and the curve drilling assembly must be reoriented when replaced in the borehole.
Another problem with surveying short radius curves is that they are often drilled through the side wall of steel casing. The proximity to the casing precludes the use of any surveying instrument that utilizes magnetic techniques to determine the borehole direction. This is true even if the drill string is tripped out of the hole. Thus, the surveying must be accomplished with "gyro" type surveying instruments which are more expensive and time consuming to operate than magnetic instruments. There are no "gyro" instruments that are small enough to pass through the inside diameter of the wiggly pipe.
Therefore, there is a need for an apparatus and method which will allow a surveying instrument, logging instrument, or other downhole tool to be positioned in the deviated section or short radius curve of a borehole without requiring the use of the drill string to move the instrument into the deviated section; which is non-magnetic i.e., which will not interfere with magnetic instruments; which does not use flexible or "wiggly" pipe sections to move the tool into the deviated section; which is light in weight; which is relatively inexpensive; and which can be "tripped" or moved in and out of the borehole quickly (compared to tripping the drill string in and out of the borehole). There is also a need for an apparatus and method which will allow a surveying instrument, logging instrument, or other downhole tool to be passed through the bore of a flexible drill pipe, particularly the wiggly pipe used for drilling short radius curves, and which will therefore not require the removal of the drill string from the borehole each time a survey, log, or other downhole operation is desired; which will allow the use of non-magnetic survey tools; which will allow surveys to be taken immediately above the drill bit as the drilling of the deviated section of the borehole progresses without removing the drill string and drill bit from the borehole; and which will allow the downhole reference point of the curve drilling assembly to be used for the survey without disturbing or requiring reestablishment of the reference point when the survey assembly is removed from the borehole.
Further, there is a need for a non-magnetic type surveying technique for use inside the drill string without removing it from the borehole when the bit is near steel casing.